Towards a Natural Theory of Dark Energy: Supersymmetric Large Extra Dimensions

TL;DR

The paper tackles the cosmological constant problem by contrasting the evidence for Dark Energy and Dark Matter with the severe naturalness challenges of scalar-field models. It then advocates Supersymmetric Large Extra Dimensions (SLED) as a concrete framework in which a classical self-tuning mechanism cancels large brane tensions via 6D bulk dynamics, while quantum bulk effects generate a small residual vacuum energy compatible with the observed Dark Energy density. The approach yields distinctive, testable predictions across gravity (micron-scale deviations and scalar-tensor behavior), cosmology (time-dependent Dark Energy from a slowly evolving radion), and particle physics (missing-energy signals from bulk emission at colliders). If realized, SLED tightly links the tiny Dark Energy scale to the geometry of large extra dimensions and opens multiple avenues for falsifiable experiments in both astrophysical and high-energy contexts.

Abstract

The first part of this article summarizes the evidence for Dark Energy and Dark Matter, as well as the naturalness issues which plague current theories of Dark Energy. The main point of this part is to argue why these naturalness issues should provide the central theoretical guidance for the search for a successful theory. The second part of the article describes the present status of what I regard as being the best mechanism yet proposed for addressing this issue: Six-dimensional Supergravity with submillimetre-sized Extra Dimensions (Supersymmetric Large Extra Dimensions, or SLED for short). Besides summarizing the SLED proposal itself, this section also describes the tests which this model has passed, the main criticisms which have been raised, and the remaining challenges which remain to be checked. The bottom line is that the proposal survives the tests which have been completed to date, and predicts several distinctive experimental signatures for cosmology, tests of gravity and for accelerator-based particle physics.

Figures (4)

• Figure 1: The energy density of radiation (dotted), Dark Matter (dashed) and Dark Energy (solid) as a function of universal scale factor, $a$, from the nucleosynthesis epoch until now with the convention that the present epoch is $a=1$.
• Figure 2: Loop graphs which can generate large quantum contributions to the scalar potential. The dashed line represents a graviton, a dotted line represents the Dark Energy field and a solid line represents a particle of mass $M$.
• Figure 3: A scale-invariant potential with a flat direction along which $V=0$ and scale invariance is broken.
• Figure 4: A scale-invariant potential along which the flat direction is lifted, with scale invariance not broken at the minimum.